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Selling on the weekend and providing a picture of the product are among the factors that increase prices in internet auctions, say US researchers.

In an effort to demystify the dynamics of buying and selling online, Charles Wood from the University of Notre Dame and the Robert Kauffman from the University of Minnesota have tracked transactions in rare coins on the internet auction site eBay for the past two years.

They presented their findings this week at the meeting of the Institute for Operations Research and the Management Sciences (INFORMS) in Florida.

Operations researchers use mathematics and science to improve decision-making, management, and operations in a host of fields. In the case of Kauffman and Wood, that meant examining 7,362 different coin transactions between 1999 and 2001.

They believe their results, though still preliminary, are also relevant to other auction items.

The researchers identified four factors that increased purchase price: selling on the weekend, providing a picture of the product, greater experience on the part of the seller, and the length of the auction.

The importance of each of these factors, however, has changed over time as the overall market depth of online auctions  that is, the number of buyers and sellers participating  has grown.

Items sold on the weekend commanded about a 2 per cent higher price than items sold during the week, the researchers found.

Dubbed the 'weekend effect', this has remained fairly constant over the two-year period, suggesting it is a phenomenon of human nature rather than of the market. It may indicate that people are willing to pay more when they have more time to consider the purchase, Kaufmann and Wood suggest.

Items accompanied by an online photo attracted an even higher premium of 11.3 per cent  and the 'picture effect' has become more important over time, with the premium having increased from 5.7 per cent since 1999.

The rise may reflect an ever-more-demanding consumer, who now expects sellers to use advanced technologies to present as much information as possible, say the researchers.

A third price boost was provided by the 'reputation score effect', where more experienced sellers have an advantage over less experienced sellers.

And finally, the study found that auctions that lasted longer resulted in higher prices because they attracted more bidders.

The 'auction length effect', however, has become less important over time. As the overall size of the online market has increased, so has the number of bidders in shorter auctions, and therefore their ability to attract high prices.